The present invention relates generally to surface emitting lasers and more specifically to the integration of surface emitting lasers and a photodiode on a substrate.
Surface emitting lasers have many advantages over conventional edge emitting lasers including a simpler manufacturing process, a single longitudinal mode of operation, higher coupling efficiency and lower cost. In order to maintain constant power output of surface emitting lasers in an optical communication system, the output power of the SEL (surface emitting laser) must be monitored. As photodiodes are typically used to monitor the output power of a surface emitting laser, it is desirable to integrate the monitoring photodiode and SEL on a single substrate.
One solution for integration of a monitoring photodiode and a surface emitting laser is reported in the reference by G. Hasnian et al., "Monolithic Integration of Photodiode with Vertical Cavity Surface Emitting Laser,", Electronics Letters (27) 18, p 1630, 1991, which describes a growth of a PIN diode on the p-type mirror region of a top-emitting surface emitting laser (SEL). FIG. 1 shows a PIN photodiode structure 100 grown on the p-type mirror region 102 of a top-emitting SEL 104. The PIN photodiode 100 is comprised of a p-type region 102, an i-type absorption region 106, and an n-type region 108. To operate the photodiode 100 as a power monitoring device, the photodiode 100 is reverse biased by applying a positive voltage to n-contact 110 while p-contact 112 is connected to ground. A negative bias is applied to n-contact 114 to forward bias the SEL 104. The absorption layer 106 absorbs a portion of the light output by the SEL 104. Knowing the amount of light absorbed by the absorption layer 110, the output power of the SEL 104 can be determined.
Although the photodiode 100 gives good performance, manufacturing complexity is increased by the steps of adding additional epitaxial layers necessary to form the i-type absorption region 106 and the n-type region 108 of the photodiode. Further, the additional epitaxial layers necessary for photodiode formation must be etched to the surface of the p-type mirror region to form a p-contact. The etch to the surface of the p-type mirror region of the SEL leaves the sidewalls of the i-type absorption layer 106 and the n-type region 108 of the photodiode exposed. The exposed epitaxial layers are subject to oxidation which decreases device reliability.
A second alternative solution for integration of a photodiode and a surface emitting laser is reported in the article "Detector-enclosed Vertical Cavity Surface Emitting Lasers", Electronics Letters (29)5 p. 466, 1993 by K. D. Choquette et al., which describes a top emitting SEL where the photodiode is formed in a concentric ring around the SEL. The concentric ring photodiode is positioned around 40 microns away from the SEL. Light from the SEL is scattered in free space and is captured by the concentric absorption region. Similar to the embodiment shown in FIG. 1, the photodiode structure described in the article "Detector-enclosed Vertical Cavity Surface Emitting Lasers", exposes the sidewalls of epitaxial layers Specifically, the process for photodiode formation includes an etch step which exposes the n-type and p-type mirror regions of the SEL resulting in oxidation of the sidewalls of the exposed regions.
A method of integrating a photodiode and surface emitting laser which minimizes process complexity and minimizes exposed epitaxial layers is needed.